The present invention relates to a method and a device for monitoring the alignment of a measuring instrument, wherein the monitoring device is equipped with an inclination sensor. The invention further relates to a measuring instrument, specifically a balance, which is equipped with the monitoring device.
Measuring instruments, in particular gravimetric measuring instruments such as for example thermo-gravimetric instruments, gravimetric moisture-determination instruments, or balances, have to meet special requirements in regard to how they are set up at the place where they are used. This applies in particular to balances equipped with a weighing cell and a load receiver which are used for the gravimetric measurement of weights. Ideally, a balance is set up in a position where the measurement axis of the balance—i.e., the axis that should coincide with the line of action of the weight force of a weighing object to be measured—runs in the direction of the gravity field. This ideal position can also be called the reference position of the balance. If the measurement axis of the balance, which is normally perpendicular to the plane of the weighing pan, is inclined at an angle relative to the gravity field, the weighing result will have a value that reflects the actual weight of the object multiplied by the cosine of the angle of inclination.
This is the reason why balances that conform to the requirements for official certification are often equipped with an inclination sensor and with a leveling device that allows the balance to be set to the reference position which is indicated by the inclination sensor. The sensor signals of an electrical inclination sensor, which are for example delivered to a display unit, indicate by how much the sensor axis which is normally aligned with the measurement axis of the balance deviates from the direction of the gravity field. The leveling device, which includes for example two axially adjustable feet of the balance, allows a deviation of the sensor axis and thus of the measurement axis from the gravity axis to be corrected.
A balance with an electrical inclination sensor is disclosed in DE 32 34 372 A1 [1], where the sensor signals are not used to correct the position of the balance, but to digitally correct the inclination-dependent error of the balance. According to [1], the inclination sensor can have either a pendulum mass or a partially filled liquid container with a gas bubble, where the position of the movable element is detected by optical or inductive means.
An electrical inclination sensor consisting of a sprit level with a container holding a partial filling of an electrically conductive liquid with a gas bubble is disclosed in JP 61 108 927 A2 [2]. According to [2], this inclination sensor is used in a balance and triggers an acoustical alarm when the out-of-level condition reaches a limit value.
The principal construction of a spirit level is described in detail in DE 38 00 155 A1 [3]. With the concept proposed in [3] the angle of inclination can be read directly from the spirit level.
An arrangement is disclosed in JP 58033114 with a spirit level that has a light-emitting element on one side and a plurality of optical sensors on the other side. A light-transmitting colored liquid is enclosed in a hemispherical container in such a way that a bubble is formed. The container itself is enclosed in a cube-shaped transparent housing. The optical sensors are arranged on the outside of the cube-shaped housing. The light rays from the light-emitting element pass through the liquid and the bubble and fall on the optical sensors. If the spirit level is put in an inclined position, the bubble moves out of place and the signals of the optical sensors change. Both the angle and the direction of the inclination are detected with this device.
An optoelectronic inclination measuring system with a deformable pendulum configured as a dual parallel spring linkage that cooperates with an emitter and a receiver unit is described in DE 43 16 046 C1. In addition to a sensor diode, there can be reference diodes arranged on the receiver side to detect and compensate the effects of undesirable extraneous factors such as changes in temperature and voltage.
An inclinometer of very high sensitivity which works in two dimensions is described in DE 199 31 679 A1. It has a spirit level with a light source arranged at the underside. An optoelectronic sensor, preferably of a type that is based on CCD (charge-coupled device) technology, extends across the top of the spirit level.
If the balance is not provided with an automatic inclination-monitoring arrangement, it is a requirement in weighing procedures that are relevant to product quality to inspect the spirit level and verify the leveled position of the balance before the weighing process is started. However, this rule is not always adhered to in practice. With automatic monitoring, on the other hand, there can be failures in the monitoring device. It is possible on the one hand that an alarm is triggered although the angle of inclination has not passed its prescribed limit, and on the other hand it can occur that the limit is exceeded even for a long period of operation without triggering an alarm. Both kinds of errors can have serious consequences. With the first kind of error, the false alarms which are also referred to as “false positives” can cause an unnecessary interruption of the measurement or production process. With the second kind of errors, also referred to as “false negatives”, the measurements and/or production processes continue in spite of the fact that the prescribed tolerance limits have been exceeded.